An undercover cop (Leonardo DiCaprio) infiltrates the gang of a Boston crime lord (Jack Nicholson), but one of the boss’s henchmen (Matt Damon) has infiltrated the Boston police department—and the two are in a race to smoke each other out. William Monahan’s script (adapted from the 2002 Hong Kong shoot-’em-up Infernal Affairs) sets up a nifty cat-and-mouse premise, and director Martin Scorsese, at the top of his game, stages some great set pieces. (And it’s good to see DiCaprio and Damon together, proving that they are not in fact the same person.) But Monahan and Scorsese can’t find an ending; the characters just give up and start blasting away at each other. Nicholson’s showy star turn misfires; he turns the film into a Jack Nicholson movie the way the older John Wayne used to turn Westerns into John Wayne movies.